1. Field of the Invention
The present invention generally relates to graphics processing and more specifically to approximation of stroked higher-order curved segments by quadratic segments.
2. Description of the Related Art
Path rendering is a style of resolution-independent two-dimensional (2D) rendering, often called “vector graphics,” that is the basis for a number of important rendering standards such as PostScript, Java 2D, Apple's Quartz 2D, OpenVG, PDF, TrueType fonts, OpenType fonts, PostScript fonts, Scalable Vector Graphics (SVG) web format, Microsoft's Silverlight and Adobe Flash for interactive web experiences, Open XML Paper Specification (OpenXPS), drawings in Office file formats including PowerPoint, Adobe Illustrator illustrations, and more.
Path rendering is resolution-independent meaning that a scene is described by paths without regard to the pixel resolution of the framebuffer. This is in contrast to the resolution-dependent nature of so-called bitmapped graphics. Whereas bitmapped images exhibit blurred or pixilated appearance when zoomed or otherwise transformed, scenes specified with path rendering can be rendered at different resolutions or otherwise transformed without blurring the boundaries of filled or stroked paths.
Sometimes the term vector graphics is used to mean path rendering, but path rendering is a more specific approach to computer graphics. While vector graphics could be any computer graphics approach that represents objects (typically 2D) in a resolution-independent way, path rendering is a much more specific rendering model with salient features that include path filling, path stroking, path masking, compositing, and path segments specified as Bèzier curves.
FIG. 1A is a prior art scene composed of a sequence of paths. In path rendering, a 2D picture or scene such as that shown in FIG. 1A is specified as a sequence of paths. Each path is specified by a sequence of path commands and a corresponding set of scalar coordinates. Path rendering is analogous to how an artist draws with pens and brushes. A path is a collection of sub-paths. Each sub-path (also called a trajectory) is a connected sequence of line segments and/or curved segments. Each sub-path may be closed, meaning the sub-path's start and terminal points are the same location so the stroke forms a loop; alternatively, a sub-path can be open, meaning the sub-path's start and terminal points are distinct.
When rendering a particular path, the path may be filled, stroked, or both. As shown in FIG. 1A, the paths constituting the scene are stroked. When a path is both filled and stroked, typically the stroking operation is done immediately subsequent to the filling operation so the stroking outlines the filled region. Artists tend to use stroking and filling together in this way to help highlight or offset the filled region so typically the stroking is done with a different color than the filling.
FIG. 1B is the sequence of paths shown in FIG. 1A with only filling. Filling is the process of coloring or painting the set of pixels “inside” the closed sub-paths of a path. Filling is similar to the way a child would “color in between the lines” of a coloring book. If a sub-path within a path is not closed when such a sub-path is filled, the standard practice is to force the sub-path closed by connecting its end and start points with an implicit line segment, thereby closing the sub-path, and then filling that resulting closed path.
While the meaning of “inside a path” generally matches the intuitive meaning of this phrase, path rendering formalizes this notion with what is called a fill-rule. The intuitive sense of “inside” is sufficient as long as a closed sub-path does not self-intersect itself. However if a sub-path intersects itself or another sub-path or some sub-paths are fully contained within other sub-paths, what it means to be inside or outside the path needs to be better specified.
Stroking is distinct from filling and is more analogous to tracing or outlining each sub-path in a path as if with a pen or marker. Stroking operates on the perimeter or boundary defined by the path whereas filling operates on the path's interior. Unlike filling, there is no requirement for the sub-paths within a path to be closed for stroking. For example, the curve of a letter “S” could be stroked without having to be closed though the curve of the letter “O” could also be stroked.
FIG. 1C is a prior art scene composed of the sequence of paths from FIG. 1A with the stroking from FIG. 1A and the filling from FIG. 1B. FIG. 1C shows how filling and stroking are typically combined in a path rendering scene for a complete the scene. Both stroking and filling are integral to the scene's appearance.
Traditionally, graphics processing units (GPUs) have included features to accelerate 2D bitmapped graphics and three-dimensional (3D) graphics. In today's systems, nearly all path rendering is performed by a central processing unit (CPU) performing scan-line rendering with no acceleration by a GPU. GPUs do not directly render curved primitives so path rendering primitives such as Bèzier segments and partial elliptical arcs must be approximated by lots of tiny triangles when a GPU is used to render the paths. Constructing the required tessellations of a path that is approximated by many short connected line segments can create a substantial CPU burden. The triangles or other polygons resulting from tessellation are then rendered by the GPU. Because GPUs are so fast at rasterizing triangles, tessellating paths into polygons that can then be rendered by GPUs is an obvious approach to GPU-accelerating path rendering.
Tessellation is a fragile, often quite sequential, process that requires global inspection of the entire path. Tessellation depends on dynamic data structures to sort, search, and otherwise juggle the incremental steps involved in generating a tessellation. Path rendering makes this process considerably harder by permitting curved path segments as well as allowing path segments to self-intersect, form high genus topologies, and be unbounded in size.
A general problem with using a GPU to render paths is unacceptably poor antialiasing quality when compared to standard CPU-based methods. The problem is that GPUs rely on point sampling for rasterization of triangular primitives with only 1 to 8 samples (often 4) per pixel. CPU-based scan-line methods typically rely on 16 or more samples per pixel and can accumulate coverage over horizontal spans.
Animating or editing paths is costly because it requires re-tessellating the entire path since the tessellation is resolution dependent, and in general it is very difficult to prove a local edit to a path will not cause a global change in the tessellation of the path. Furthermore, when curved path segments are present and the scaling of the path with respect to pixel space changes appreciably (zooming in say), the curved path segments may need to be re-subdivided and re-tessellation is likely to be necessary.
Additionally, compositing in path rendering systems typically requires that pixels rasterized by a filled or stroked path are updated once-and-only-once per rasterization of the path. This requirement means non-overlapping tessellations are required. So for example, a cross cannot be tessellated as two overlapping rectangles but rather must be rendered by the outline of the cross, introducing additional vertices and primitives. In particular, this means the sub-paths of a path cannot be processed separately without first determining that no two sub-paths overlap. These requirements, combined with the generally fragile and sequential nature of tessellation algorithms make path tessellation particularly expensive. Because of the expense required in generating tessellations, it is very tempting and pragmatic to cache tessellations. Unfortunately such tessellations are much less compact than the original path representations, particularly when curved path segments are involved. Consequently, a greater amount of data must be stored to cache paths after tessellation compared with storing the paths prior to tessellation.
Conventional stroking has been performed by approximating paths into sub-pixel linear segments and then tracing the segments with a circle having a diameter equal to a stroke width. Offset curves are generated at the boundary of the stroked path. These offset curves are typically of much higher degree of complexity compared with the linear segments that are traced to generate the stroked path. Determining whether or not each pixel is inside or outside of a stroked path to generate the stroking is mathematically complex. Identification of the pixels to be stroked is equivalent to identifying pixels that are within half of the stroke width of any point along the path to be stroked. More specifically, the pixels to be stroked are within half of the stroke width measured along a line that is perpendicular to the tangent of the path segment being stroked.
The tangent of a sub-path is not necessarily well-defined at junctions between path segments. So additional rules are needed to determine what happens at and in the vicinity of such junctions as well as what happens at the terminal (start and end) points of sub-paths. Therefore stroking specifies further stroking rules to handle these situations.
In standard path rendering systems, paths are specified as a sequence of cubic and quadratic (non-rational) Bèzier curve segments, partial elliptical arcs, and line segments. While more mathematically complex path segments representations could be used to specify paths, in practice, existing standards limit themselves to the aforementioned path segment types.
Path filling and stroking use the same underlying path specification. For filling, this means the resulting piece-wise boundaries to be filled may be up to third-order (in the case of cubic Bèzier segments) or rational second-order (in the case of partial elliptical arcs). Filling these curved boundaries of Bèzier curves and arcs is clearly harder than filling the standard polygonal primitives in conventional polygonal 2D or 3D rendering where the boundaries (edges) of the polygonal primitives (usually triangles) are all first-order, being linear segments, and often required to be convex. Filling (and stroking) are also harder than conventional line and convex polygon rasterization because paths are unbounded in their complexity whereas line segments and triangles are defined by just 2 or 3 points respectively. A path may contain just a single path segment or it could contain thousands or more.
The boundaries of stroked paths are actually substantially higher order than the third-order segments. The offset curve of non-rational (second-order) quadratic and (third-order) cubic Bèzier curves are eighth- and tenth-order curves respectively. This high order makes exact determination and evaluation of the resulting offset curves for such Bèzier segments intractable for use in direct rendering. In other words, it is quite unreasonable to try to determine exactly the boundary representation of such offset curves and then simply fill them. For this reason, various techniques have been developed to approximate offset curves with sequences of Bèzier, arc, or line segments. These approximate stroke boundaries may then be filled.
FIG. 1D illustrates prior art exterior stroke offset curves for various stroke widths of a generating path 200. Observe in FIG. 1D how as the stroke width radius increases, the respective offset curves 207 for each different stroke width exhibit self-intersections 222 and cusps 223. The exterior stroke offset curves are higher-order curves compared with the generating path 200. FIG. 1E illustrates interior stroke offset curves for various stroke widths of a generating path 221. The interior stroke bounding curves are higher-order curves compared with the generating path 221. Observe in FIG. 1E how offset curves with a small radius leave a topological hole inside the generating path 221. As the radius increases with each wider radius, the hole splits into two holes. The largest radius shown fills in the hole completely. These changes in the genus of the region bounded by offset curves and the emergence of tangent discontinuities on the boundary of wide offset curves illustrate some of the difficulties associated with exact rasterization of stroked paths.
The idea that stroking is “harder” than filling is a bit unintuitive when filling and stroking are considered on an intuitive, artistic level. An artist typically thinks of stroking as a form of sketching or outlining whereas filling requires “coloring in between the lines.” In typical rasterized path rendering scenes, most of pixels tend to be painted by filling rather than stroking so there is a sense that more effort is expended to perform the filling simply because more pixels were painted by filling.
This intuition seems to be further validated when one appreciates that evaluating the fill-rule required for proper filling requires a global view of the entire path. Just because a pixel appears to be inscribed within a particular loop of a path does not mean the pixel should be painted because the path might contain another loop with the opposite winding order that all inscribes that pixel. Certainly there are very intricate paths where determining whether a pixel filled by such an intricate path is quite involved; however most paths, in practice, are often reasonably simple (meaning non-self-intersecting and topologically genus zero).
However this naïve intuition that filling might be easier is misleading; proper stroking is hard because of the mathematical complexity of the boundary of a path's stroke compared to a path's fill. While approximations to the actual stroke boundary can reduce this complexity, such approximations have associated costs due to inaccuracy and the resulting expansion in the number of primitives that must be both stored and processed to render such approximated strokes. For example, the stroke of a quadratic Bèzier segment can be represented with just the segment's 3 control points (along with the per-path stroke width) whereas an approximation of this stroked boundary with line segments might require dozens or even hundreds of triangles to tessellate approximately the stroked region. Indeed the quality of such tessellations depends on the projection of the curved segment to screen-space; this means rendering the same stroked curve at different resolutions would necessitate different tessellations.
Accordingly, what is needed in the art is an improved system and method for approximating stroked higher-order curved segments using quadratic segments.